Serde is a powerful framework that enables serialization libraries to generically serialize Rust data structures without the overhead of runtime type information. In many situations, the handshake protocol between serializers and serializees can be completely optimized away, leaving Serde to perform roughly the same speed as a hand written serializer for a specific type.
Documentation is available at http://serde-rs.github.io/serde/serde
The simplest way to make a type serializable is to use the serde_macros
syntax extension, which comes with a #[derive(Serialize, Deserialize)]
annotation, which automatically generates implementations of
Serialize
and
Deserialize
for the annotated type:
```rust
extern crate serde;
...
struct Point { x: i32, y: i32, } ```
Serde bundles a high performance JSON serializer and deserializer, serde::json, which comes with the helper functions tostring and fromstr that make it easy to go to and from JSON:
```rust use serde::json;
...
let point = Point { x: 1, y: 2 }; let serializedpoint = json::tostring(&point).unwrap();
println!("{}", serialized_point); // prints: {"x":1,"y":2}
let deserializepoint: Point = json::fromstr(&serialized_point).unwrap(); ```
serde::json also supports a generic Value type, which can represent any JSON value. Also, any Serialize and Deserialize can be converted into a Value with the methods tovalue and fromvalue:
```rust let point = Point { x: 1, y: 2 }; let pointvalue = json::tovalue(&point).unwrap();
println!("{}", point_value.find("x")); // prints: Some(1)
let deserializepoint: Point = json::fromvalue(point_value).unwrap(); ```
Under the covers, Serde extensively uses the Visitor pattern to thread state between the Serializer and Serialize without the two having specific information about each other's concrete type. This has many of the same benefits as frameworks that use runtime type information without the overhead. In fact, when compiling with optimizations, Rust is able to remove most or all the visitor state, and generate code that's nearly as fast as a hand written serializer format for a specific type.
To see it in action, lets look at how a simple type like i32 is serialized.
The
Serializer
is threaded through the type:
rust
impl serde::Serialize for i32 {
fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
where S: serde::Serializer,
{
serializer.visit_i32(*self)
}
}
As you can see it's pretty simple. More complex types like BTreeMap need to
pass a
MapVisitor
to the
Serializer
in order to walk through the type:
```rust
impl(&self, serializer: &mut S) -> Result<(), S::Error>
where S: Serializer,
{
serializer.visit_map(MapIteratorVisitor::new(self.iter(), Some(self.len())))
}
}
pub struct MapIteratorVisitor
impl
impl
#[inline]
fn len(&self) -> Option<usize> {
self.len
}
} ```
Serializing structs follow this same pattern. In fact, structs are represented as a named map. It's visitor uses a simple state machine to iterate through all the fields:
```rust struct Point { x: i32, y: i32, }
impl serde::Serialize for Point {
fn serialize(&self, serializer: &mut S) -> Result<(), S::Error>
where S: serde::Serializer
{
serializer.visitnamedmap("Point", PointMapVisitor {
value: self,
state: 0,
})
}
}
struct PointMapVisitor<'a> { value: &'a Point, state: u8, }
impl<'a> serde::ser::MapVisitor for PointMapVisitor<'a> {
fn visit(&mut self, serializer: &mut S) -> Result
Deserialization is a little more complicated since there's a bit more error
handling that needs to occur. Let's start with the simple i32
Deserialize
implementation. It passes a
Visitor to the
Deserializer.
The Visitor
can create the i32 from a variety of different types:
```rust
impl Deserialize for i32 {
fn deserialize
struct I32Visitor;
impl serde::de::Visitor for I32Visitor { type Value = i32;
fn visit_i16<E>(&mut self, value: i16) -> Result<i16, E>
where E: Error,
{
self.visit_i32(value as i32)
}
fn visit_i32<E>(&mut self, value: i32) -> Result<i32, E>
where E: Error,
{
Ok(value)
}
...
```
Since it's possible for this type to get passed an unexpected type, we need a way to error out. This is done by way of the Error trait, which allows a Deserialize to generate an error for a few common error conditions. Here's how it could be used:
```rust ...
fn visit_string<E>(&mut self, _: String) -> Result<i32, E>
where E: Error,
{
Err(serde::de::Error::syntax_error())
}
...
```
Maps follow a similar pattern as before, and use a MapVisitor to walk through the values generated by the Deserializer.
```rust
impl
pub struct BTreeMapVisitor
impl
impl
fn visit_unit<E>(&mut self) -> Result<BTreeMap<K, V>, E>
where E: Error,
{
Ok(BTreeMap::new())
}
fn visit_map<V_>(&mut self, mut visitor: V_) -> Result<BTreeMap<K, V>, V_::Error>
where V_: MapVisitor,
{
let mut values = BTreeMap::new();
while let Some((key, value)) = try!(visitor.visit()) {
values.insert(key, value);
}
try!(visitor.end());
Ok(values)
}
}
```
Deserializing structs goes a step further in order to support not allocating a
String to hold the field names. This is done by custom field enum that
deserializes an enum variant from a string. So for our Point example from
before, we need to generate:
```rust enum PointField { X, Y, }
impl serde::Deserialize for PointField {
fn deserialize
impl serde::de::Visitor for FieldVisitor {
type Value = Field;
fn visit_str<E>(&mut self, value: &str) -> Result<PointField, E>
where E: serde::de::Error
{
match value {
"x" => Ok(Field::X),
"y" => Ok(Field::Y),
_ => Err(serde::de::Error::syntax_error()),
}
}
}
deserializer.visit(FieldVisitor)
}
} ```
This is then used in our actual deserializer:
```rust
impl serde::Deserialize for Point {
fn deserialize
struct PointVisitor;
impl serde::de::Visitor for PointVisitor { type Value = Point;
fn visit_map<V>(&mut self, mut visitor: V) -> Result<Point, V::Error>
where V: serde::de::MapVisitor
{
let mut x = None;
let mut y = None;
loop {
match try!(visitor.visit_key()) {
Some(Field::X) => { x = Some(try!(visitor.visit_value())); }
Some(Field::Y) => { y = Some(try!(visitor.visit_value())); }
None => { break; }
}
}
let x = match x {
Some(x) => x,
None => try!(visitor.missing_field("x")),
};
let y = match y {
Some(y) => y,
None => try!(visitor.missing_field("y")),
};
try!(visitor.end());
Ok(Point{ x: x, y: y })
}
} ```